From what I have seen, husk of the pit will drop from any hive activity. Step 1, kill approximately 100 knights. Step 2, visit the speaker then Eris morn Step 3, complete The Abyss, kill crota on 390, kill omnigul. That's as far as I have got? Anyone got any further?
what you have to kill in the first step varies depending on the gun. if you inspect it, the upgrade node will tell you what you need to kill. as far as i know, its either thralls, knights or wizards.